Thermoelectric cooling and heating has been known for many years; however, its use has not been cost competitive except for limited applications because of the lack of thermoelectric materials having the needed thermoelectric properties.
A good thermoelectric material is measured by its "figure of merit" or Z, defined as EQU Z=S.sup.2 /.rho.K
where S is the Seebeck coefficient, .rho. is the electrical resistivity, and K is the thermal conductivity. The Seebeck coefficient is further defined as the ratio of the open-circuit voltage to the temperature difference between the hot and cold junctions of a circuit exhibiting the Seebeck effect, or EQU S=V/(T.sub.h -T.sub.c).
Therefore, in searching for a good thermoelectric material, we look for materials with large values of S, and low values of .rho.0 and K.
Thermoelectric materials currently in use today include the materials listed below with their figures of merit shown:
______________________________________ Peak Zeta, Thermoelectric Material Z (at temperature shown) ZT ______________________________________ Lead telluride 0.9 .times. 10.sup.-3 /.degree.K. at 500.degree. K. 0.9 Bismuth telluride 3.2 .times. 10.sup.-3 /.degree.K. at 300.degree. K. 1.0 Silicon germanium 0.8 .times. 10.sup.-3 /.degree.K. at 1100.degree. K. 0.9 ______________________________________
Workers in the thermoelectric have been attempting too improve the figure of merit for the past 20-30 years with not much success. Most of the effort has been directed to reducing the lattice thermal conductivity (K) without adversely affecting the electrical conductivity. Experiments with superlattice quantum well materials have been underway for several years. These materials were discussed in an paper by Gottfried H. Dohler which was published in the November 1983 issue of Scientific American. This article presents an excellent discussion of the theory of enhanced electric conduction in superlattices. These superlattices contain alternating conducting and barrier layers. These superlattice quantum well materials are crystals grown by depositing semiconductors in layers whose thicknesses is in the range of a few to up to about 100 angstroms. Thus, each layer is only a few atoms thick. There has been speculation that these materials might be useful as thermoelectric materials. (See articles by Hicks, et al and Harman published in Proceedings of 1992 1st National Thermoelectric Cooler Conference Center for Night Vision & Electro Optics, U.S. Army, Fort Belvoir, Va. FIG. 1 has been extracted from the Hicks paper and is included herein as prior art. It projects theoretically very high ZT values as the layers are made progressively thinner.) The idea being that these materials might provide very great increases in electric conductivity without adversely affecting Seebeck coefficient or the thermal conductivity. Harmon of Lincoln Labs, operated by MIT has claimed to have produced a superlattice of layers of (Bi,Sb) and Pb(Te,Se). He claims that his preliminary measurements suggests ZTs of 3 to 4. However, the present inventors have demonstrated that high ZT values can definitely be achieved with Si/Si.sub.0.8 Ge.sub.0.2 superlattice quantum wells. Most of the efforts to date with superlattices have involved alloys which are known to be good thermoelectric materials for cooling, many of which are difficult to manufacture as superlattices because the stoichiometry of the alloys have to be very carefully controlled which is very difficult in vapor deposition techniques such as molecular beam epitaxy.
What is needed are thermoelectric materials with improved ZT values which permit a simplified manufacturing process.